Constant-bitrate (CBR) rate control is very useful for real-time video transmission. However, it is difficult to realize a good CBR rate control in an MPEG4-AVC (ISO/IEC 14496-10) video codec. Generally, in an MPEG4-AVC codec both, bitrate and distortion depend on a quantization parameter QP that controls the degree of quantisation. By adjusting QP one can get a trade-off between rate-QP and distortion-QP. However, most rate control algorithms either do not achieve a good visual quality or suffer from rather large controlling errors. In the Joint Video Team (JVT) reference source codes, use of the rate control algorithms proposed by
Z. Li, W. Gao, F. Pan, S. Ma, K. P. Lin, G. Feng, X. Lin, S. Rahardja, H. Lu and Y. Lu, “Adaptive Rate Control with HRD Consideration”, document JVT-H014, 8th meeting, Geneva, May 2003, denoted. [1],
and by Z. G. Li, F. Pan, K. P. Lim, X. Lin and S. Rahardja, “Adaptive Rate Control for H.264”, ICIP, 2004, denoted [2], has been recommended by the JVT committee.
Zhihai He has proposed in “ρ-Domain Rate-Distortion Analysis and Rate Control for Visual Coding and Communication”, PhD Dissertation, University of California, Santa Barbara, June 2001, denoted [3], a new model that achieves a good performance for H.263 and MPEG4-2 (ISO/IEC 14496-2) codecs. The parameter ‘ρ’ represents the percentage of zeros among the quantized transform coefficients. He found a basically linear relationship between the value ‘ρ’ and the real bit rate because the percentage of zeros plays an important role in determining the final bit rate. He assumed that the distribution of residual coefficients complies with either Gaussian distribution or Laplace distribution, so that the linear relationship or model can be derived as follows: The Laplace distribution for the residual image isR(ρ)=2*log2e(1−ρ)+O([1−ρ]3)  (1)wherein ‘O’ is the higher-order infinite small.
The Gaussian distribution for the original image is
                                          R            ⁡                          (              ρ              )                                =                                    log              2                        ⁢                                          1                +                                  x                  a                                                            1                +                                  x                  a                                -                                  2                  ⁢                  x                                +                                                      (                                          1                      -                      a                                        )                                    ⁢                                      (                                          1                      +                                              x                        a                                                              )                                    ⁢                  x                  ⁢                                                                          ⁢                  ln                  ⁢                                                                          ⁢                  x                                                                    ⁢                                  ⁢        where        ⁢                                  ⁢                              a            =                          0.5                              0.5                +                b                                              ,                      x            =                          1              -              ρ                                                          (        2        )            and where ‘a’ and ‘b’ are statistical parameters of the original image.
The relation between bitrate R and percentage ρ of quantized transform zero values can be approximated by:R(ρ)=Θ*(100−ρ)  (3)wherein ρ is expressed in a % value and Θ is a constant representing the slope of the linear relationship.
The final goal is to find the relation between the percentage ρ of zero coefficients P and the quantization parameter QP. According to Z. He, there is a one-to-one mapping between QP and ρ. Therefore the rate/distortion functions R(QP) and D(QP) in the QP-domain can be mapped in the ρ-domain denoted as R(ρ) and D(ρ), respectively.
For the H.263 video compression the relation between the percentage ρ of zero coefficients P and the quantization parameter QP it is given by:
                              P          ⁡                      (            QP            )                          =                                                            P                ⁡                                  (                  QP                  )                                            intra                        -                                          P                ⁡                                  (                  QP                  )                                            inter                                =                                                    ∑                                  x                  ∈                  IntraMB                                            ⁢                              (                                  if                  ⁡                                      (                                                                                          x                                                                    ≺                                              2                        ⁢                        QP                                                              )                                                  )                                      +                                          ∑                                  x                  ∈                  InerMB                                            ⁢                              (                                  if                  ⁡                                      (                                                                                          x                                                                    ≺                                              2.5                        ⁢                        QP                                                              )                                                  )                                                                        (        4        )            
This linear ρ-domain model has a good performance with respect to controlling errors and visual quality in MPEG4-2.
A good rate control provides good coding/decoding properties such as small controlling errors and a good visual quality, which both are important for practical MPEG4-AVC applications. However, the quantization processing and the percentage ρ of zeros among the quantized transform or DCT transform (discrete cosine transform) coefficients in MPEG4-AVC is different from that used in MPEG4-2. The original Z. He ρ-domain model cannot be used for an MPEG4-AVC codec because of the conflict between rate/distortion optimization (RDO) and rate control, which both are dependent from the quantization step size.
The above-mentioned rate control algorithms [1] and [2] cause controlling errors in picture sequences including e.g. flashes or in/out fades. The visual quality is unstable because of usage of too much different QPs for different (temporally or spatially adjacent) macroblocks. Moreover, the rate control in [2] employs the information from previous units or scenes so that it is not a suitable solution for coding abrupt changes of scene content in a video sequence.